Don't come to FL June, July, or August. Hot, humid, bugs.
Spring break, the weather is nice, but places like Disney get PACKED. We have the Disney silver pass, and that's part of the block-out dates, due to the number of visitors they get.
IMHO, late Fall is the best time to visit. Temperature/humidity are better, less bugs, less crowds.
If you are thinking about staying at Fort Wilderness, make sure you have reservations as many months in advance as possible. Fills up quickly.